Many obligate bacterial membrane proteins hijack human cellular pathways by mimicking or manipulating host machinery. The goal of this research is to investigate the structure and dynamics of bacterial outer membrane proteins and their interactions with host receptors. Specifically, research is focused on the outer membrane opacity-associated proteins (Opa) from Neisseria gonorrhoeae and Neisseria meningitides, which induce engulfment of the bacterium in non-phagocytic cells by binding to host receptors. Opa proteins bind to various host receptors and are classified into two families based on host receptor selectivity. The larger class, OpaCEA, bind to carcinoembryonic antigen-like cellular adhesion molecules (CEACAMs), and the smaller class, OpaHS, bind to two different receptors; the heparansulfate proteoglycan receptors (HSPGs) directly and indirectly to integrin receptors via a heparin- mediated interaction with fibronectin or vibironectin. Opa proteins are integral outer membrane proteins and predicted to have an eight-stranded 2-barrel fold. Two of the extracellular loops (HV1 and HV2) have the most sequence variation between Opa proteins and determine the host receptors specificity. Not only do the HV loops discriminate between HSPG and CEACAM receptors, but OpaCEA proteins can be further divided into subgroups based on the selective binding to four of the seven CEACAM receptors. Using nuclear magnetic resonance, electron paramagnetic resonance, isothermal titration calorimetry, and mutagenesis, the molecular determinants of these interactions will be determined. The results will provide insight into the pathogenesis of Neisseria gonorrhoeae and Neisseria meningitides and, therefore, the potential for the rational design of novel antibiotics. In addition, the reconstituted Opa proteins may be useful for vaccine development. However, the most novel application of this research lies in the ability of Opa proteins to target host receptors specifically via three different mechanisms to induce endocytosis in non-phagocytic cells. This ability may be useful for liposome pharmaceutical carriers. The potential ability of liposome encapsulated therapeutics (e.g. enzymes, inhibitors, and peptides) to enter the cytoplasm of living cells and possibly tissue selectively is of crucial importance to the treatment of many diseases. Understanding the molecular determinants of the three Opa-mediated entry mechanisms may facilitate the development of liposome delivery mechanisms.